Imaging lenses ranging from microscope objectives to satellite-based cameras are physically limited in the total number of features they can resolve. These limitations are a function of the point-spread function size of the imaging system and the inherent aberrations across its image plane field of view. Referred to as the space-bandwidth product, the physical limitation scales with the dimensions of the lens but is usually on the order of 10 megapixels regardless of the magnification factor or numerical aperture (NA). While conventional imaging systems may be able to resolve up to 10 megapixels, there is typically a tradeoff between point-spread function and field of view. For example, certain conventional microscope objectives may offer a sharp point-spread function across a narrow field of view, while others imaging systems with wide-angle lenses can offer a wide field of view at the expense of a blurry point-spread function.
Traditionally, the resolution of an image sensor, such as in a digital camera, determines the fidelity of visual features in the resultant images captured by the image sensor. However, the resolution of any image sensor is fundamentally limited by geometric aberrations in the lens or lenses used to focus light onto the image sensor. This is because the number of resolvable points for a lens, referred to as the SBP, is fundamentally limited by geometrical aberrations. While CMOS and CCD technologies have been demonstrated having image sensors with pixels in the 1 micron (μm) range, it remains a challenge to design and manufacture lenses which have the resolving power to match the resolution of such image sensors.
Certain interferometric synthetic aperture techniques try to increase spatial-bandwidth product. Most of these interferometric synthetic aperture techniques include setups that record both intensity and phase information using interferometric holography such as off-line holography and phase-shifting holography. Interferometric holography has its limitations. For example, interferometric holography recordings typically use highly coherent light sources. As such, the constructed images typically suffer from coherent noise sources such as speckle noise, fixed pattern noise (induced by diffraction from dust particles and other optical imperfections in the beam path), and multiple interferences between different optical interfaces. Thus the image quality is typically worse than from a conventional microscope. On the other hand, using off-axis holography sacrifices spatial-bandwidth product (i.e., reduces total pixel number) of the image sensor. In addition, interferometric imaging techniques may be subject to uncontrollable phase fluctuations between different measurements. Hence, accurate a priori knowledge of the sample location may be needed to set a reference point in the image recovery process. Another limitation is that many of these interferometric imaging systems require mechanical scanning to rotate the sample and thus precise optical alignments, mechanical control at a sub-micron level, and associated maintenances are required by these systems. In terms of spatial-bandwidth product, these interferometric imaging systems may present little to no advantage as compared with a conventional microscope. Previous lensless microscopy such as in-line holography and contact-imaging microscopy also present drawbacks. For example, conventional in-line holography does not work well with contiguous samples and contact-imaging microscopy requires a sample to be in close proximity to the sensor.
A high spatial-bandwidth product is very desirable in imaging applications such as microscopy for biomedical imaging such as used in pathology, haematology, phytotomy, immunohistochemistry, and neuroanatomy. For example, there is a strong need in biomedicine and neuroscience to image large numbers of histology slides for evaluation.